The invention relates to circuit arrangement and to a method for current measurement of an electric current in a measurement path which corresponds to an electric current in a power path. In particular, the invention relates to a circuit arrangement and a method for current measurement for transistors having a sense terminal.
Although present invention together with the problem addressed thereby is explained on the basis of an IGBT (insulated-gate bipolar transistor) having a sense terminal, it is also applicable to any other transistors or circuits composed of transistors in which a sense terminal is provided.
The document U.S. Pat. No. 5,877,617 A discloses a circuit arrangement in which a load is supplied with a first current. Said first current is provided via a first transistor in a power path. Furthermore, the circuit arrangement comprises a second transistor, which is connected in parallel with the first transistor and which provides in a measurement path a second current proportional to the first current in the power path. The second current in that case serves for monitoring and evaluation of the current flow through the load.
The document U.S. Pat. No. 5,200,878 A discloses a circuit arrangement comprising an insulated-gate bipolar transistor (IGBT) embodied with an additional sense terminal, which provides a sense current proportional to the emitter current. The ratio of the two currents is approximately constant under specific conditions and depends with sufficient accuracy only on the technical implementation of the transistor. The sense terminal in that case typically serves as a measurement path and the emitter terminal as a power path. On account of the proportionality, the emitter current can be determined in principle by means of the measurement of the sense current. Furthermore, the document describes a drive circuit of the gate electrode of such an IGBT comprising a sense terminal.
Typical circuit arrangements for measuring the sense current provide for the sense current to be evaluated over the entire duration of the switching period of the IGBT. In this case, the components of such a circuit arrangement have to be dimensioned for the maximum duration of the switching period of the IGBT and in particular for the average currents and dropping powers and that occur here. By way of example, this may necessitate a cooling of the components used.